A quantitative PID tuning procedure is a step-by-step approach leading directly to a set of numerical values to be used in a PID controller. These procedures may be split into two categories: open loop and closed loop. An “open loop” tuning procedure is implemented with the controller in manual mode: introducing a step-change to the controller output and then mathematically analyzing the results of the process variable response to calculate appropriate PID settings for the controller to use when placed into automatic mode.
A “closed loop” tuning procedure is implemented with the controller in automatic mode: adjusting tuning parameters to achieve an easily-defined result, then using those PID parameter values and information from a graph of the process variable over time to calculate new PID parameters.
Quantitative PID tuning got its start with a paper published in the November 1942 Transactions of the American Society of Mechanical Engineers written by two engineers named Ziegler and Nichols. “Optimum Settings For Automatic Controllers” is a seminal paper, and deserves to be read by every serious student of process control theory. That Ziegler’s and Nichols’ recommendations for PID controller settings may still be found in modern references more than 60 years after publication is a testament to its impact in the field of industrial control.
Although dated in its terminology and references to pneumatic controller technology (some controllers mentioned as not even having adjustable proportional response, and others as having only discrete degrees of reset adjustment rather than continuously variable settings!), the PID algorithm described by its authors and the effects of P, I, and D adjustments on process control behavior are as valid today as they were then.
This article is devoted to a discussion of quantitative PID tuning procedures in general, and the “Ziegler-Nichols” methods in specific. It is the opinion of this author that the Ziegler-Nichols tuning methods are useful primarily as historical references, and indeed suffer from serious practical impediments. The most serious reservation I have with the Ziegler-Nichols methods (and in fact any algorithmic procedure for PID tuning) is that these methods tend to absolve the practitioner of responsibility for understanding the process they intend to tune.
Any time you provide people with step-by-step instructions to perform complex tasks, there will be a great many readers of those instructions tempted to mindlessly follow the instructions, even to their doom. PID tuning is one of these “complex tasks,” and there is significant likelihood for a person to do more harm than good if all they do is implement a step-by-step approach rather than understand what they are doing, why they are doing it, and what it means if the results do not meet with satisfaction. Please bear this in mind as you study any PID tuning procedure, Ziegler-Nichols or otherwise.
Types of Quantitative PID tuning procedures